Air Pollution Impacts

Joshua Tree National Park

Natural and scenic resources in Joshua Tree National Park (NP) are susceptible to the harmful effects of air pollution. Nitrogen, ozone, and fine particles impact natural resources such as plant communities and scenic resources such as visibility. Click on the tabs below to learn more about air pollutants and their impacts on natural and scenic resources at Joshua Tree NP.

Nitrogen & Sulfur

Ozone

Visibility

Nitrogen deposition at Joshua Tree NP in California has promoted growth of non-native, invasive grasses, such as cheatgrass (above).

Nitrogen and sulfur compounds deposited from air pollution can harm soils, vegetation, and the rare surface waters located throughout the park. Arid ecosystems are particularly vulnerable to changes caused by nitrogen deposition. Nitrogen deposition may disrupt soil nutrient cycling and alter plant communities. Invasive grasses tend to thrive in areas with high nitrogen deposition, displacing native vegetation adapted to low nitrogen conditions.

Ozone and Public Health Concerns

Ground-level ozone concentrations at Joshua Tree NP are among the highest recorded in national parks and frequently exceed the National Ambient Air Quality Standards set by the U.S. Environmental Protection Agency to protect public health.

Naturally-occurring ozone in the upper atmosphere absorbs the sun’s harmful ultraviolet rays and helps to protect all life on earth. However, in the lower atmosphere, ozone is an air pollutant, forming when nitrogen oxides from vehicles, power plants, and other sources combine with volatile organic compounds from gasoline, solvents, and vegetation in the presence of sunlight. In addition to causing respiratory problems in people, ozone can injure plants. Ozone enters leaves through pores (stomata), where it can kill plant tissues, causing visible injury, or reduce photosynthesis, growth, and reproduction.

Joshua Tree NP experiences high ozone concentrations, with peak levels and cumulative doses that are some of the highest in the NPS. These concentrations are known to cause injury to sensitive plants. While the generally dry conditions in the park are likely to limit ozone uptake by plants and subsequent injury, a wet year or strong summer monsoon season may increase the risk of ozone injury.

Under certain conditions, ozone levels at Joshua Tree NP are sufficiently high to induce foliar injury to ozone-sensitive plant species such as the blue elderberry (Sambucus mexicana).

There are several ozone-sensitive plants in the park including Rhus trilobata (skunkbush sumac), Salix gooddingii (Goodding’s willow), and Sambucus mexicana (blue elderberry). Limited assessments in the park have not documented ozone injury to vegetation growing naturally in the field (Temple 1989); however, no assessment has been made of other ozone effects such as growth effects. A study of Rhus trilobata conducted in a park biomonitoring plot demonstrated that under irrigated conditions the plants showed typical ozone injury symptoms (Temple 1989), demonstrating that ozone levels are sufficiently high in Joshua Tree NP to induce foliar injury, and possibly reduced growth effects, under certain conditions (Sullivan et al. 2001 [pdf, 6.3 MB]).

Many visitors come to Joshua Tree NP to enjoy the spectacular vistas, including that of the Mexican border from the mile-high vantage point of Keys View. Unfortunately, such vistas are often obscured by haze caused by fine particles in the air. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition contribute to this haze and visibility impairment. Organic compounds, soot, and dust reduce visibility as well.

Visibility effects at Joshua Tree NP include:

Reduced visibility on many days due to haze;

Reduction of the average natural visual range from about 160 miles (without the effects of pollution) to about 60 miles because of pollution at the park;

Reduction of the visual range from about 120 miles to below 35 miles on high pollution days.